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Abstract

The purpose of this investigation was to study the behavior and develop an ultimate strength model for composite beams with rectangular web openings. Six composite beams with concentric rectangular web openings were tested to failure. Varying moment to shear ratios were used to help develop and verify the ultimate strength model. One steel beam was tested to demonstrate the contribution of the concrete to the capacity of composite beams. Two steel sections were used, while the concrete slab size was held constant for all beams. Three elastic tests were performed on each beam before it was tested to failure. An ultimate strength model was developed to predict the strength of composite beams at the web openings in the form of moment-shear interaction diagrams. The steel is modeled as an elastic-perfectly plastic material. The yield stress of the steel is defined as a function of the assigned shear stress according to the von Mises yield criterion. The concrete is modeled for ultimate strength behavior. Concrete compressive strength is also defined as a function of the assigned shear stress, based on experimental results. Strain compatibility between the concrete and steel is assumed. The model is compared with experimental results and is used to study the effect of key parameters (material properties, opening size, and opening eccentricity) affecting the strength of composite beams with web openings. A simplified design interaction procedure is presented to conservatively guide the placement of web openings in composite beams. Based on the experimental study, it is clear that web openings can greatly reduce the strength of composite beams. It appears that the secondary bending has a sizable effect on beam behavior. The concrete in composite beams contributes, not only, to the flexural strength, but, also, to the shear capacity of the beams at web openings. The ultimate strength of composite beams at web openings is governed by the failure of the concrete. The ultimate strength model satisfactorily predicts the strength of the experimental beams.